As a spindle motor of this type, there have been known spindle motors disclosed in JP-A 2004-248337, JP-A 2002-354742, and the like.
In recent years, there have been successively developed AV products and home electric products each of which includes a hard disc drive. A use form of such products tends to be changed from a stationary type to a portable type. From a request of portability, there has been required a small-sized, thin spindle motor. In addition, from a request of cost reduction, there has been also required reduction in the number of components.
Particularly, in a spindle motor used in a hard disc drive, a hydrodynamic bearing is adopted as a bearing for the purpose of achieving high accuracy, silent performance, and extended service life of the spindle motor.
Hereinafter, description will be given of a structure and operation of a conventional spindle motor (see JP-A 2002-354742).
As illustrated in FIG. 7, a conventional spindle motor includes a shaft 20, a flange 21, a sleeve 22, a thrust plate 23, an adhesive 24, a rotor hub 25, a magnet 26, a stator core 27, a coil 28, a base internal cylindrical part 29, an attraction plate 30, and a base member 31.
First, the flange 21 is fixed to the shaft 20 by means of laser welding or the like. Next, the shaft 20 is inserted into and fitted to the sleeve 22. Thereafter, the thrust plate 23 is brought into contact with the flange 21, and a tip end 32 of the sleeve 22 is fixed to the thrust plate 23 by means of caulking or the like. Further, the thrust plate 23 is sealed with the adhesive 24. Thus, a bearing unit is assembled. Then, the bearing unit is filled with lubricating fluid (not illustrated), so that a hydrodynamic bearing is obtained. The rotor hub 25 to which the magnet 26 is fixedly bonded by an adhesive or the like is fixed to the bearing unit assembled as described above; thus, a rotor unit is obtained.
On the other hand, the attraction plate 30 is fixedly bonded to the base member 31 by an adhesive or the like. The stator core 27 having the coil 28 wound therearound is fixedly bonded to the base internal cylindrical part 29; thus, a stator unit is obtained. Finally, the rotor unit is fitted to the stator unit and, then, they are fixedly bonded to each other by an adhesive or the like. In the spindle motor configured as described above, when a current is applied to the coil 28 such that a rotational magnetic field is generated at an outer periphery of the stator core 27, the rotor unit starts to rotating. Then, a radial bearing is formed by dynamic pressure generating grooves cut on an outer periphery of the shaft 20 or an inner circumference of the sleeve 22, a thrust main bearing is formed by dynamic pressure generating grooves cut on a lower face of the flange 21 or an upper face of the thrust plate 23, and a thrust sub bearing is formed by dynamic pressure generating grooves cut on an upper face of the flange 21 or a lower face of the sleeve 22. Thus, the rotor unit rotates with respect to the stator unit in a non-contact manner. The magnet 26 generates an attraction force in relation to the attraction plate 30; therefore, a displacement of the rotor unit in an axial direction is not largely changed by a posture of the spindle motor.
JP-A 2004-248337 discloses a spindle motor wherein a stator core is directly fitted to a sleeve for the purpose of downsizing.
However, even when the size of the spindle motor having the aforementioned conventional configuration is simply reduced, a required torque cannot be attained. In addition, a dimension of the magnet is made relatively large and the number of windings of the coil is increased. Consequently, magnetic saturation occurs in the stator core, a torque waveform is distorted, and vibration and noise are generated. As a result, it is impossible to sufficiently reduce the size of the spindle motor under present circumstances.